My girl friend broke up with me just because she want to become a nun (sister) ?

I have a wonderful relationship with my girlfriend, it has been 4 years of relationship, and we just celebrated our 4th anniversary yesterday, then she gave a heart attack by asking me to let her go.

Reason become she want to become a nun. She said she has lost interest in building a family with love one, she lost interest on world stuff, she said she never realize how far we have gone. and she said still struggling to let go of everything. and she said i am most dearly to her heart, that is why she is telling me, non of her family & friends know about this.

i am so shocked to hear this, she even plan to marry me and organise a family with 3 kids few months ago. now i really do not know what to do. i am hopeless. and for what i understand about her, she have been stress from work, and both our parent do not agreed with our relationship. i dont know is it the reason she want to excape from stressful lifestyle.

i am still not ready to let her go. heart broken, bleeding, pain, sad, disappointed, emotional... all come in once. anyone can assist me? what should i do?